1. Field of the Invention
This invention relates to a method of controlling an output of an internal combustion engine and/or a variable displacement hydraulic pump driven by the engine. More particularly, it relates to a control method which enables the efficient operation of an internal combustion engine or a pump driven by the engine, or both, while maintaining the fuel consumption of the engine at a low level.
2. Description of the Prior Art
There is known an internal combustion engine of the type which is controlled in accordance with a specific pattern irrespective of any change in the torque requirement of a variable displacement hydraulic pump (hereinafter referred to simply as the variable pump) which is driven by the engine, i.e., its built-in displacement multiplied by its output pressure. The torque requirement of the variable pump is altered by a mode selector control device to maintain the fuel consumption of the engine at a low level. See copending U.S. patent application Ser. No. 717,197 filed by T. Akiyama et al., now U.S. Pat. No. 4,637,781, dated Jan. 20, 1987.
The engine has a fuel injection device including a mechanical all-speed type governor. The curve b in FIG. 1 is a governor control curve and each of curves a.sub.1 to a.sub.5 shows a specific amount of fuel consumed by the engine in such a way that its fuel consumption decreases in the order of curves a.sub.5 to a.sub.1. The fuel consumption of the engine is always fixed at a specific point on the governor control curve b. For example, it is shown by the curve a.sub.3 at a rated point C on the curve b.
The work (mode) of the variable pump which is driven by the engine having such governor control characteristics may, for example, be variable in three stages, i.e., a high-load mode M.sub.1, a medium-load mode M.sub.2 and a low-load mode M.sub.3, as shown in FIG. 2. Then, the engine is controlled by the mechanical governor for operation at points C (rated point), S and L on the governor control curve b, respectively. When the mode of the variable pump is altered, the engine has an output torque which greatly differs from one mode to another, though the number of revolutions of the engine is maintained substantially at a constant level.
As a result, the torque requirement of the variable pump also differs greatly from one mode to another, as shown in FIG. 2. As the variable pump is so designed as to show the best efficiency in one of its modes, for example, M.sub.1, its efficiency greatly differs from one mode to another. Therefore, it has the disadvantage of failing to utilize the output of the engine effectively in either of the modes other than M.sub.1.
Each of curves in FIG. 2 is a curve of equal variable pump efficiency. The efficiency of the pump is shown as increasing with a decrease in the radius of curvature of the curves.
Moreover, the control of the engine by the conventional mechanical governor has the disadvantage that the engine consumes a large amount of fuel at a low load, as shown at point L in FIG. 1.